This invention generally relates to equipment for filtering liquids and more particularly to filter assemblies for use with radioactive by-products of nuclear power plants.
Normal operation of a nuclear power plant requires the continuous processing of thousands of gallons of liquids which contain radioactive solids in suspension. These liquids must be filtered so that the liquids may be safely reused and the relatively high radioactivity levels may be confined to isolated portions of the power plant. The most common filtering equipment in use today employs disposable cartridge filters which entrap the radioactive solids during the course of the filtering action and which must be periodically replaced.
The conventional filter assembly for use with disposable cartridges is loaded and unloaded from above the filtering chamber. The filter assembly is usually located below the floor of the power plant and covered with a shield plug forming part of the floor. Since the cartridges become highly radioactive during use, the filter assembly is located within concrete shielded cubicles having a thickness of several feet on all sides. Removal of the spent cartridge begins with removal of the shield plug using an overhead hoist, laying the shield plug aside, and exposing the filter assembly beneath the floor. The operator must remain behind a shield wall to prevent becoming overly exposed to the radiation yielded by the assembly. From behind the shield, the operator must remotely loosen and lift the assembly closure cover. In many plants this operation is remotely performed with long handled tools or hooks and poles. Once the closure cover is opened and open-bottomed shield transfer cask weighing several tons is positioned above the assembly. A cartridge grappling tool is then lowered through the cask to grasp the cartridge. The dripping cartridge is withdrawn into the transfer cask. Upon verification that the cartridge is in the transfer cask, the cask is removed from the access opening, attached to a base, and transferred to a disposal area. It can be appreciated that reloading top loaded filter assemblies is very hazardous and therefore great care must be taken to protect the operator.
Bottom loaded filter assemblies have been proposed for use in nuclear power plants, but these have also been unsatisfactory. Such a filter system has a housing with a bolted-on bottom hatch which closes the bottom of the housing and which functions when in place to mount a filter cartridge inside the housing. The cartridge drops into a shipping cask or receptacle when the hatch is disconnected from the housing and slid transversely thereto. Such prior art units have two major disadvantages. The first is that the housing is typically surrounded by a very thick concrete shield which occupies a large space in the reactor building. Secondly, the means by which the hatch is located under and attached to the housing necessitates many manual operations to unbolt the hatch, align the replacement cartridge filter within the housing, and reattach the hatch. These operations are performed inside the concrete shielding and are very hazardous. Since the liquid to be filtered is under high pressure, ranging from 200 to over 2,000 psi, a very tight seal between the hatch and the housing must be made. Unfastening the many bolts that are provided to keep the filter assembly leak-tight either exposes the workmen to prolonged periods of irridiation, or if performed remotely, cause excessive delay in replacing a filter cartridge.